In an effort to decrease automobile weight and increase energy efficiency, many automobile manufacturers are employing automobile parts that are fabricated from lightweight materials, such as plastic materials (e.g., thermoplastics and thermosets). One such class of plastic materials is generally known as thermoplastic polyolefins (TPO's), such as various grades of polyethylene or polypropylene. Another such class would consist of ABS or ABS/polycarbonate blends.
Some of these parts are produced by thermoforming, e.g., vacuum forming, which is generally defined as a process wherein a heated, and thus softened, amount of plastic material (typically in the form of a sheet) is molded into the desired shape through vacuum suction of the warmed plastic onto a pre-formed mold.
Thermoforming can be used for many thicknesses of plastic sheets and can provide great strength in its finished moldings. Fairly complex moldings can be achieved with thermoforming. However, these plastic materials do not typically have a desirable finish (or prematurely lose that finish when exposed to the elements) for use with automotive applications. Furthermore, these plastic materials do not possess sufficient wall thickness. For example, certain conventional automotive components are thermoformed, trimmed and then injection molded.
Various approaches have been taken to providing a high quality automotive paint-like finish to lightweight parts, such as molded plastic parts. One approach that has received considerable attention uses a surfacing film system having a decorative layer (e.g., a preformed colorant or paint-like film) that can be used to impart a surface effect (e.g., color or other visual pattern) to a thermoformed part. These surfacing film systems are generally referred to as paint or color-containing films. An example of this process can be found in U.S. Pat. No. 5,215,826 to Shimanski et al., the entire specification of which is expressly incorporated herein by reference.
By way of a non-limiting example, paint or color-containing films, especially those used in producing colored automotive components, can be comprised of a decorative layer (e.g., paint, ink, or other colorant), an optional adhesive layer (e.g., a heat-activated adhesive), a preferably scratch resistant optional top clear coat layer, and an optional removable casting base (e.g., a polyester-based sheet). These types of paint or color-containing films are readily commercially available from Avery Dennison Corp. (Pasadena, Calif.), Soliant L.L.C. (Lancaster, S.C.) and Dorrie International (Farmington Hills, Mich.). These paint or color-containing films are generally available in a wide range of colors, including solid metallic colors, and are primarily used in a number of automotive applications.
Additionally, automotive component manufacturers have also used other types of color-containing films, such as mold-in-color (MIC) films. One particular MIC film currently being used is a MIC ionomer film readily commercially available from Mayco Plastics, Inc. (Sterling Heights, Mich.) under the trade name FORMION. The MIC ionomer film typically consists of four discrete layers with a back molded (e.g., injection molded) thermoplastic polyolefin substrate. The layers typically consist of a clear ionomer layer, a colored ionomer layer, an adhesive layer, and a backing layer.
Regardless of the type of paint or color-containing film used, it is sometimes necessary to employ a removable release or masking layer to protect the surface of the component to be thermoformed, due in part to the particular processing parameters encountered during the thermoforming process. This is especially true of thick sheet polyolefin thermoforming. Thick sheet thermoforming typically employs sheets having a thickness in the range of about 0.06 to about 0.5 inches.
Typically, when the thermoforming process is completed, the component is removed from the mold surface and the release layer is then removed. Alternatively, the release layer can be left in place, for example, until the component reaches its final destination, whereupon the release layer can then be removed, thus protecting the outer surface of the component.
Unfortunately, many conventional release layer materials have several significant disadvantages, such as relatively low tensile strength, especially at the elevated temperatures typically associated with conventional thermoforming techniques, as well as undesirable bag/sag/drape control characteristics. Accordingly, when the TPO substrate is heated, regardless of whether a paint or color-containing film is present or not, the release layer exhibits significant bag/sag/drape control problems which are manifested in the TPO substrate billowing out from the frame bracket. When the TPO substrate is removed from the oven and placed over the mold face, the draping TPO material tends to gather and clump when the suction force is applied to the mold. Thus, the finished thermoformed part has significant surface irregularities and cannot be used, thus raising production costs and causing production delays.
Accordingly, there exists a need for new and improved support film systems, especially for use with paint or color-containing films, wherein the support film systems exhibit relatively high tensile strength, especially at the elevated temperatures typically associated with conventional thermoforming techniques, as well as relatively good bag/sag/drape control characteristics. Additionally, these new and improved support film systems should preserve, or at least prevent the loss of gloss characteristics of the paints films, if used, or aid in the thermoformability of the underlying plastic substrates, if paint or color-containing films are not used.